1. Field of the Invention
The present invention relates to implantable medical devices and more particularly to an enclosure structure which may be used for implantable medical devices such as: heart pacemakers, pacer/cardioverter/defibrillator and drug administration devices.
2. Description of the Prior Art
The earliest pacemakers typified by U.S. Pat. No. 3,053,356 to Wilson Greatbatch were manufactured of discrete electronic components which were potted in an epoxy resin material and then coated with silicone rubber. Epoxy is permeable to gases and permits battery gases to escape and moisture to enter the device. While operating in the human body such structures became completely perfused with water vapor and reliable operation required that no inorganic salts be present within the device, otherwise electrical short circuits could occur. The Medtronic model 5850 pacemaker is an example of this form of construction.
Continued progress in the development of heart pacemakers resulted in packaging concepts in which the electronic circuitry portion of the pacemaker was enclosed in a hermetically sealed container which was attached to external batteries. The batteries and hermetically sealed package would be potted in epoxy to form the pacemaker. This packaging strategy permitted the mercury-zinc cells to vent their gas while retaining the sensitive electronic components in a hermetic environment which excluded moisture. The Medtronic Xytron pacer is an example of this form of construction.
Development of the solid state lithium iodide battery permitted the inclusion of the electrochemical cell within a hermetic structure. A typical form of modern pacemaker construction is depicted in U.S. Pat. No. 4,314,562 where a hermetically sealed battery is included within a hermetically sealed pacemaker enclosure.
The pacemaker enclosure itself is formed by can halves, which form a "clam shell" assembly. The can halves are welded together along the seam to form the pacemaker pulse generator. Electrical communication to the stimulating leads is done through a plastic connector module fixed to the outer surface of the can and a collection of "feedthroughs" which permit electrical communication with the container but which retain hermeticity of the pacer.
A number of variations on this packaging strategy are known in the art as well. And although conventional construction techniques set forth above have produced reliable long lived pacemakers capable of outlasting their recipients there is a continuing need to improve enclosures for use in pacemakers as well as other implantable medical devices. For example, current construction techniques involve complex assembly operations which increase costs. Current construction techniques require the application of insulating anti-stimulation coatings on the exterior surface of the pacemaker to prevent muscle stimulation. Current construction practices also limit design freedom and do not readily accommodate lead wrap in the pacemaker pocket.
Other problems presented by prior art structures include: the inclusion of weld rings to permit protection of the internal components during butt welding operations; the requirement of cosmetic buffing operations to improve the appearance and biocompatibility of the enclosure; the orientation of, and the number of, feedthroughs required to provide external connections to the implanted device. Many of these features also lead to complex assembly requirements which increases costs and lowers yield from the manufacturing process.